1. Field of the Invention
The disclosure relates in general to a composite material and method of manufacturing the same, and more particularly to a gradient composite material and method of manufacturing the same.
2. Description of the Related Art
Plastic sheet, formed by injection molding or cell casting, can produce a variety of objects by different thermoplastic, thermosetting, and photocuring methods. Those plastic materials are further subjected to surface treatment, such as the secondary processing, to meet the requirements of the physical/chemical properties of the objects (such as wear resistance, electrical conductivity, thermal conductivity, anti-fouling etc.). However, the secondary processing would increase the cost and cause the problem of interface compatibility, such as adhesion or coefficient of thermal expansion (CTE) difference, etc. Typically, the inorganic materials, such as metals, ceramics, metal oxides and alloys, are used for improving or changing the physical properties of the surface of the plastic materials. However, large amounts of the inorganic materials added into the resin would increase the viscosity of the resin, which is unfavorably condition to the resin storage and processing. Also, the products (sheets or coating films) made of this resin with large amounts of the inorganic materials would be easy to crack or cause aggregation of inorganic particles, so as to affect the optical properties of the finished products.
It is desired to develop a material containing an organic base (ex: resin) and small amounts of the inorganic materials or other organic materials to possess the advantageous properties, including easy to store and process, not easy to crack, no aggregation of inorganic particles during storage stage or in the resin, and the surface properties of the finished products being improved.
Take an optical device fabrication for example. In order to increasing the light extraction efficiency of the optical device, a material is sandwiched between the lighting device and the air, wherein the refraction index of the material is in the range between the refraction indexes of the lighting device and the air. FIG. 1 simply depicts a conventional optical device with one layer of resin. Generally, the refraction index of the LED 11 is 2.5˜3.5 while the refraction index of the air 15 is 1. A total internal reflection occurs at the interface of the LED 11 and the air 15 due to the difference between the refraction indexes of the LED 11 and the air 15. Recently, the resin 13 used for encapsulating the LED 11 has a refraction index (RI) in the range of 1.4-1.5. Some special material of resin may possess a higher refraction index of about 1.6. Although the resin with high refraction index could increase the light extraction between the LED 11 and the resin 13, the difference between the refraction indexes of the resin 13 and the air 15 still exists. If the resin 13 for encapsulating the LED 11 could be designed to have a gradient refraction index, which the resin portion close to the LED 11 has higher refraction index and other portion close to the air 15 has lower refraction index, the total internal reflection would be prevented so as to effectively increase the light extraction efficiency.
Recently, the encapsulating structure with gradient refraction index applied in the optical device is achieved by laminating several molding layers with different refraction indexes, and the molding layers are arranged in an order of higher value to lower value of refraction index. FIG. 2 simply depicts another conventional optical device, having three resin layers with different refraction index for encapsulating an LED. As shown in FIG. 2, the resin layers 22-25 with different refraction indexes are orderly formed on the chip 21. The order of the refraction index of the resin layers 22-25 is n1>n2>n3>n4 (measured along the line 27). However, it is time-consuming and highly cost for fabricating the optical device with several resin layers as shown in FIG. 2. Also, it is likely to cause defects between the interfaces of the resin layers, leading to light scattering.
Moreover, in the application of display screen, the conventional optical glass is expansive, difficult to fabricate and increasing the overall weight of the finished product. Plastic sheet could be an alternative choice. Adding inorganic compounds into the plastic may increase the rigidness and hardness. However, the added contents of inorganic compounds for reaching the hardness requirement of the cover of the touch screen would degrade the ability of deformation of plastic, so that make the plastic material easy to fragile when it is subjected to pressure.